Simple system to serialize Unity objects with specified type or simple class in the same field and collection with Unity Serialization
Put SerializedInterfaceObjectReference folder into Unity project
Use SerializedInterfaceObjectReference<TInterface> to allow all Unity object types.
Use SerializedInterfaceMonoBehaviourReference<TInterface> to allow only MonoBehaviours.
Use SerializedInterfaceScriptableObjectReference<TInterface> to allow only ScriptableObjects.